Regulation of skeletal muscle creatine kinase from a hibernating mammal

Integrated transcriptomics and metabolomics reveal protective effects on heart of hibernating Daurian ground squirrels

Hibernating mammals are natural models of resistance to ischemia, hypoxia-reperfusion injury, and hypothermia. Daurian ground squirrels (spermophilus dauricus) can adapt to endure multiple torpor-arousal cycles without sustaining cardiac damage. However, the molecular regulatory mechanisms that underlie this adaptive response are not yet fully understood. This study investigates morphological, functional, genetic, and metabolic changes that occur in the heart of ground squirrels in three groups: summer active (SA), late torpor (LT), and interbout arousal (IBA). Morphological and functional changes in the heart were measured using hematoxylin-eosin (HE) staining, Masson staining, echocardiography, and enzyme-linked immunosorbent assay (ELISA). Results showed significant changes in cardiac function in the LT group as compared with SA or IBA groups, but no irreversible damage occurred. To understand the molecular mechanisms underlying these phenotypic changes, transcriptomic and metabolomic analyses were conducted to assess differential changes in gene expression and metabolite levels in the three groups of ground squirrels, with a focus on GO and KEGG pathway analysis. Transcriptomic analysis showed that differentially expressed genes were involved in the remodeling of cytoskeletal proteins, reduction in protein synthesis, and downregulation of the ubiquitin-proteasome pathway during hibernation (including LT and IBA groups), as compared with the SA group. Metabolomic analysis revealed increased free amino acids, activation of the glutathione antioxidant system, altered cardiac fatty acid metabolic preferences, and enhanced pentose phosphate pathway activity during hibernation as compared with the SA group. Combining the transcriptomic and metabolomic data, active mitochondrial oxidative phosphorylation and creatine-phosphocreatine energy shuttle systems were observed, as well as inhibition of ferroptosis signaling pathways during hibernation as compared with the SA group. In conclusion, these results provide new insights into cardio-protection in hibernators from the perspective of gene and metabolite changes and deepen our understanding of adaptive cardio-protection mechanisms in mammalian hibernators.

Purification and characterization of NADP-isocitrate dehydrogenase from skeletal muscle of Urocitellus richardsonii

NADP-dependent isocitrate dehydrogenase (NADP-IDH, EC 1.1.1.42) catalyzes the oxidative decarboxylation of isocitrate to α-ketoglutarate with the concomitant production of NADPH. NADPH plays important roles in many biosynthesis pathways, maintenance of proper oxidation-reduction balance, and protection against oxidative damage. This present study investigated the dynamic nature of NADP-IDH during hibernation by purifying it from the skeletal muscle of Richardson's ground squirrel (Urocitellus richardsonii) and analyzing its structural and functional changes in response to hibernation. Kinetic parameters of purified NADP-IDH from euthermic and hibernating ground squirrel skeletal muscle were characterized at 22 °C and 5 °C. Relative to euthermic muscle, -NADP-IDH in hibernating muscle had a higher affinity for its substrate, isocitrate at 22 °C, whereas at 5 °C, there was a significant decrease in isocitrate affinity. Western blot analysis revealed greater serine and threonine phosphorylation in hibernator NADP-IDH as compared to euthermic NADP-IDH. In addition, Bioinformatic analysis predicted the presence of 18 threonine and 21 serine phosphorylation sites on squirrel NADP-IDH. The structural and functional changes in NADP-IDH indicate the ability of the organism to reduce energy consumption during hibernation, while emphasizing increased NADPH production, and thus antioxidant activity, during torpor arousal cycles.

Natural Products Containing 'Rare' Organophosphorus Functional Groups

Phosphorous-containing molecules are essential constituents of all living cells. While the phosphate functional group is very common in small molecule natural products, nucleic acids, and as chemical modification in protein and peptides, phosphorous can form P⁻N (phosphoramidate), P⁻S (phosphorothioate), and P⁻C (e.g., phosphonate and phosphinate) linkages. While rare, these moieties play critical roles in many processes and in all forms of life. In this review we thoroughly categorize P⁻N, P⁻S, and P⁻C natural organophosphorus compounds. Information on biological source, biological activity, and biosynthesis is included, if known. This review also summarizes the role of phosphorylation on unusual amino acids in proteins (N- and S-phosphorylation) and reviews the natural phosphorothioate (P⁻S) and phosphoramidate (P⁻N) modifications of DNA and nucleotides with an emphasis on their role in the metabolism of the cell. We challenge the commonly held notion that nonphosphate organophosphorus functional groups are an oddity of biochemistry, with no central role in the metabolism of the cell. We postulate that the extent of utilization of some phosphorus groups by life, especially those containing P⁻N bonds, is likely severely underestimated and has been largely overlooked, mainly due to the technological limitations in their detection and analysis.

Temperature and serine phosphorylation regulate glycerol-3-phosphate dehydrogenase in skeletal muscle of hibernating Richardson's ground squirrels

Glycerol-3-phosphate dehydrogenase (G3PDH) bridges carbohydrate and lipid metabolism by interconverting glycerol-3-phosphate (G3P) and dihydroxyacetone phosphate (DHAP). This reversible reaction converts G3P derived from triglyceride hydrolysis to DHAP that can then enter glycolysis or gluconeogenesis and, in the reverse reaction, makes G3P for use in triglyceride biosynthesis. Small hibernating mammals rely almost exclusively on triglyceride reserves as their fuel for energy production during torpor and the recovery of glycerol after lipolysis is an important source of carbohydrate over the nonfeeding winter months. G3PDH (∼37 kDa) was purified from skeletal muscle of euthermic and hibernating Richardson's ground squirrels (Urocitellus richardsonii) using three column chromatography steps. Analysis of enzyme kinetic properties revealed that G3PDH from hibernator muscle had higher affinities for G3P and NAD at low (5 °C) assay temperature compared with high (21 or 37 °C) and a greater stability in the presence of denaturing agents (urea, guanidine hydrochloride) or high temperature (50 °C). Immunoblotting showed that hibernator muscle G3PDH had a higher phosphoserine content than the enzyme from euthermic controls and incubation studies showed that enzyme affinity for G3P changed significantly by stimulating endogenous protein kinases or phosphatases. Overall, this study suggests that the properties of ground squirrel muscle G3PDH are modulated by temperature and post-translational phosphorylation to alter enzyme function under euthermic versus hibernating states.

Effects of tetraploidy induction on rainbow trout (Oncorhynchus mykiss, Walbaum, 1792) proteome at early stages of development

The aim of the present study was to examine the effects of tetraploidy induction on proteome of rainbow trout during the early stages of development. After insemination, the eggs were incubated at 10°C for 350min. Thereafter, half of the eggs were exposed to a heat-shock of 28°C for 10min. The remainder were incubated normally and used as diploid controls. Fertilized egg specimens were selected 390min post-fertilization. Samples corresponding respectively to eyed embryos and fry stages were also taken on days 18 and 76 post-fertilization. Based on two-dimensional electrophoresis, all spots that were found to differ significantly in abundance between the untreated and heat-shock treated groups were selected for identification using MALDI-TOF/TOF mass spectrometry. Out of 19 protein spots showing altered abundance in the present study, 13 spots were successfully identified. Of the spots that were shown to change in abundance in the fertilized eggs with heat-shock treatment, three were identified as vitellogenin (spots 1, 2 and 3); while the others were creatine kinase (spot 5) and nucleoside diphosphate kinase (spot 6). All of the proteins identified in the embryos were related to vitellogenin (spots 8, 12 and 13). Among the identified spots from the fry muscle extracts, two were identified as beta-globin (spots 14 and 17); while the others were parvalbumin (spots 15 and 16) and creatine kinase (spot 19). The results obtained in our study may now set the ground for investigations on gene regulation and proteome modifications in polyploid fish.

mTORC1 is Required for Brown Adipose Tissue Recruitment and Metabolic Adaptation to Cold

In response to cold, brown adipose tissue (BAT) increases its metabolic rate and expands its mass to produce heat required for survival, a process known as BAT recruitment. The mechanistic target of rapamycin complex 1 (mTORC1) controls metabolism, cell growth and proliferation, but its role in regulating BAT recruitment in response to chronic cold stimulation is unknown. Here, we show that cold activates mTORC1 in BAT, an effect that depends on the sympathetic nervous system. Adipocyte-specific mTORC1 loss in mice completely blocks cold-induced BAT expansion and severely impairs mitochondrial biogenesis. Accordingly, mTORC1 loss reduces oxygen consumption and causes a severe defect in BAT oxidative metabolism upon cold exposure. Using in vivo metabolic imaging, metabolomics and transcriptomics, we show that mTORC1 deletion impairs glucose and lipid oxidation, an effect linked to a defect in tricarboxylic acid (TCA) cycle activity. These analyses also reveal a severe defect in nucleotide synthesis in the absence of mTORC1. Overall, these findings demonstrate an essential role for mTORC1 in the regulation of BAT recruitment and metabolism in response to cold.

Regulation of brain-type creatine kinase by AMP-activated protein kinase: interaction, phosphorylation and ER localization

AMP-activated protein kinase (AMPK) and cytosolic brain-type creatine kinase (BCK) cooperate under energy stress to compensate for loss of adenosine triphosphate (ATP) by either stimulating ATP-generating and inhibiting ATP-consuming pathways, or by direct ATP regeneration from phosphocreatine, respectively. Here we report on AMPK-dependent phosphorylation of BCK from different species identified by in vitro screening for AMPK substrates in mouse brain. Mass spectrometry, protein sequencing, and site-directed mutagenesis identified Ser6 as a relevant residue with one site phosphorylated per BCK dimer. Yeast two-hybrid analysis revealed interaction of active AMPK specifically with non-phosphorylated BCK. Pharmacological activation of AMPK mimicking energy stress led to BCK phosphorylation in astrocytes and fibroblasts, as evidenced with a highly specific phospho-Ser6 antibody. BCK phosphorylation at Ser6 did not affect its enzymatic activity, but led to the appearance of the phosphorylated enzyme at the endoplasmic reticulum (ER), close to the ER calcium pump, a location known for muscle-type cytosolic creatine kinase (CK) to support Ca²⁺-pumping.

Proteomic screening of molecular targets of crocin

Background

Traditional drug discovery approaches are mainly relied on the observed phenotypic changes following administration of a plant extract, drug candidate or natural product. Recently, target-based approaches are becoming more popular. The present study aimed to identify the cellular targets of crocin, the bioactive dietary carotenoid present in saffron, using an affinity-based method.

Methods

Heart, kidney and brain tissues of BALB/c mice were homogenized and extracted for the experiments. Target deconvolution was carried out by first passing cell lysate through an affinity column prepared by covalently attaching crocin to agarose beads. Isolated proteins were separated on a 2D gel, trypsinized in situ and identified by MALDI-TOF/TOF mass spectrometry. MASCOT search engine was used to analyze Mass Data.

Results

Part of proteome that physically interacts with crocin was found to consist of beta-actin-like protein 2, cytochrome b-c1 complex subunit 1, ATP synthase subunit beta, tubulin beta-3 chain, tubulin beta-6 chain, 14-3-3 protein beta/alpha, V-type proton ATPase catalytic subunitA, 60 kDa heat shock protein, creatine kinase b-type, peroxiredoxin-2, cytochrome b-c1 complex subunit 2, acetyl-coA acetyltransferase, cytochrome c1, proteasome subunit alpha type-6 and proteasome subunit alpha type-4.

Conclusion

The present findings revealed that crocin physically binds to a wide range of cellular proteins such as structural proteins, membrane transporters, and enzymes involved in ATP and redox homeostasis and signal transduction.

Suppression of MAPKAPK2 during mammalian hibernation

Metabolic signaling coordinates the transition by hibernating mammals from euthermia into profound torpor. Organ-specific responses by activated p38 mitogen activated protein kinase (MAPK) are known to contribute to this transition. Therefore, we hypothesized that the MAPK-activated protein kinase-2 (MAPKAPK2), a downstream target of p38 MAPK, would also be active in establishing the torpid state. Kinetic parameters of MAPKAPK2 from skeletal muscle of Richardson's ground squirrels, Spermophilus richardsonii, were analyzed using a fluorescence assay. MAPKAPK2 activity was 27.4±1.27 pmol/min/mg in muscle from euthermic squirrels and decreased by ∼63% during cold torpor, while total protein levels were unchanged (as assessed by immunoblotting). In vitro treatment of MAPKAPK2 via stimulation of endogenous phosphatases and addition of commercial alkaline phosphatase decreased enzyme activity to only ∼3-5% of its original value in muscle extracts from both euthermic and hibernating squirrels suggesting that posttranslational modification suppresses MAPKAPK2 during the transition from euthermic to torpid states. Enzyme S₀.₅ and n(H) values for ATP and peptide substrates changed significantly between euthermia and torpor, and also between assays at 22 versus 10 °C but, kinetic parameters were actually closely conserved when values for the euthermic enzyme at 22 °C were directly compared with the hibernator enzyme at 10 °C. Arrhenius plots showed significantly different activation energies of 40.8±0.7 and 54.3±2.7 kJ/mol for the muscle enzyme from euthermic versus torpid animals, respectively but MAPKAPK2 from the two physiological states showed no difference in sensitivity to urea denaturation. Overall, the results show that total activity of MAPKAPK2 is in fact reduced, despite previous findings of p38 MAPK activation, and kinetic parameters are altered when ground squirrels enter torpor but protein stability is not apparently changed. The data suggest that MAPKAPK2 suppression may have a significant role in the differential regulation of muscle target proteins when ground squirrels enter torpor.

Insights into the in vivo regulation of glutamate dehydrogenase from the foot muscle of an estivating land snail

Land snails, Otala lactea, survive in seasonally hot and dry environments by entering a state of aerobic torpor called estivation. During estivation, snails must prevent excessive dehydration and reorganize metabolic fuel use so as to endure prolonged periods without food. Glutamate dehydrogenase (GDH) was hypothesized to play a key role during estivation as it shuttles amino acid carbon skeletons into the Krebs cycle for energy production and is very important to urea biosynthesis (a key molecule used for water retention). Analysis of purified foot muscle GDH from control and estivating conditions revealed that estivated GDH was approximately 3-fold more active in catalyzing glutamate deamination as compared to control. This kinetic difference appears to be regulated by reversible protein phosphorylation, as indicated by ProQ Diamond phosphoprotein staining and incubations that stimulate endogenous protein kinases and phosphatases. The increased activity of the high-phosphate form of GDH seen in the estivating land snail foot muscle correlates well with the increased use of amino acids for energy and increased synthesis of urea for water retention during prolonged estivation.

Increased cardiac alpha-myosin heavy chain in left atria and decreased myocardial insulin-like growth factor (Igf-I) expression accompany low heart rate in hibernating grizzly bears

Grizzly bears (Ursus arctos horribilis) tolerate extended periods of extremely low heart rate during hibernation without developing congestive heart failure or cardiac chamber dilation. Left ventricular atrophy and decreased left ventricular compliance have been reported in this species during hibernation. We evaluated the myocardial response to significantly reduced heart rate during hibernation by measuring relative myosin heavy-chain (MyHC) isoform expression and expression of a set of genes important to muscle plasticity and mass regulation in the left atria and left ventricles of active and hibernating bears. We supplemented these data with measurements of systolic and diastolic function via echocardiography in unanesthetized grizzly bears. Atrial strain imaging revealed decreased atrial contractility, decreased expansion/reservoir function (increased atrial stiffness), and decreased passive-filling function (increased ventricular stiffness) in hibernating bears. Relative MyHC-α protein expression increased significantly in the atrium during hibernation. The left ventricle expressed 100% MyHC-β protein in both groups. Insulin-like growth factor (IGF-I) mRNA expression was reduced by ∼50% in both chambers during hibernation, consistent with the ventricular atrophy observed in these bears. Interestingly, mRNA expression of the atrophy-related ubiquitin ligases Muscle Atrophy F-box (MAFBx) and Muscle Ring Finger 1 did not increase, nor did expression of myostatin or hypoxia-inducible factor 1α (HIF-1α). We report atrium-specific decreases of 40% and 50%, respectively, in MAFBx and creatine kinase mRNA expression during hibernation. Decreased creatine kinase expression is consistent with lowered energy requirements and could relate to reduced atrial emptying function during hibernation. Taken together with our hemodynamic assessment, these data suggest a potential downregulation of atrial chamber function during hibernation to prevent fatigue and dilation due to excessive work against an optimally filled ventricle, a response unpredicted by the Frank-Starling mechanism.

AMPK and ACCchange with fasting and physiological condition in euthermic and hibernating golden-mantled ground squirrels (Callospermophilus lateralis)

AMP-activated protein kinase (AMPK) is a cellular energy sensor that responds to low endogenous energy by stimulating fatty acid oxidation (through inactivation of acetyl-CoA carboxylase (ACC)) and food intake. Fasting generally stimulates phosphorylation of AMPK (pAMPK) and ACC (pACC), but it is unclear how AMPK and ACC react to a long-term fast (i.e. hibernation). We performed Western blots for total and pAMPK and pACC on tissues from a species of hibernator (Callospermophilus lateralis) after short-term summer fasting (1-5 days) and long-term winter fasting (3 months). Winter animals were sacrificed during hibernation at low body temperature (torpid, T(b)~5°C) or at normal high T(b)(euthermic, T(b)~37°C). We found a general increase in pAMPK in most tissues (liver, muscle, and white adipose tissue (WAT), but not hypothalamus) and pACC in all tissues after a short-term summer fast. Response of AMPK and ACC to a long-term winter fast differed by tissue-in liver, there was no difference in total or pAMPK or pACC between groups, but in muscle, WAT and BAT, euthermic GMGS had lower relative abundance of pAMPK and pACC than torpid animals. Therefore, AMPK may be an important energy sensor at all points in hibernator's circannual cycles of food intake and T(b).

A reference growth curve for nutritional experiments in zebrafish (Danio rerio) and changes in whole body proteome during development

Zebrafish is one of the most used vertebrate model organisms in molecular and developmental biology, recently gaining popularity also in medical research. However, very little work has been done to assess zebrafish as a model species in nutritional studies in aquaculture in order to utilize the methodological toolbox that this species represents. As a starting point to acquire some baseline data for further nutritional studies, growth of a population of zebrafish was followed for 15 weeks. Furthermore, whole body proteome was screened during development by means of bi-dimensional gel electrophoresis and mass spectrometry. Fish were reared under best practice laboratory conditions from hatching until 103 days post-fertilization (dpf) and regularly fed ad libitum with Artemia nauplii from 12 dpf. A growth burst occurred within 9-51 dpf, reaching a plateau after 65 dpf. Fork length and body weight were significantly lower in males than in females from 58 dpf onwards. Proteomics analysis showed 28 spot proteins differently expressed through development and according to sex. Of these proteins, 20 were successfully identified revealing proteins involved in energy production, muscle development, eye lens differentiation, and sexual maturation. In summary, zebrafish exhibited a rapid growth until approximately 50 dpf, when most individuals started to allocate part of the dietary energy intake for sexual maturation. However, proteomic analysis revealed that some individuals reached sexual maturity earlier and already from 30 dpf onwards. Thus, in order to design nutritional studies with zebrafish fed Artemia nauplii, it is recommended to select a period between 20 and 40 dpf, when fish allocate most of the ingested energy for non-reproductive growth purposes.

Relationship between proteome changes of Longissimus muscle and intramuscular fat content in finishing pigs fed conjugated linoleic acid

The present experiment was conducted to determine proteome changes in Longissimus muscle of finishing pigs fed conjugated linoleic acid (CLA), in association with alteration of intramuscular fat content. Previously, seventy-two Duroc × Landrace × Large White gilts (approximately 60 kg) had been fed maize–soyabean meal-based diets with 0, 12·5 and 25 g CLA/kg diet. The CLA contained 369·1 mg/g cis-9, trans-11 CLA, 374·6 mg/g trans-10, cis-12 CLA and 53·7 mg/g other isomers. Six pigs per treatment were slaughtered when they reached a body weight of approximately 100 kg. Data published from a previous experiment demonstrated that supplementation with 12·5 or 25 g CLA/kg diet increased intramuscular fat content (P < 0·05). The present study investigated the proteome changes in Longissimus muscle of control and pigs supplemented with 25 g CLA/kg diet. CLA significantly influenced the abundance of proteins related to energy metabolism, fatty acid oxidation and synthesis, amino acid metabolism, defence, transport and other miscellaneous processes (P < 0·05). The increase in intramuscular fat content was positively correlated with the increased abundance of carbonic anhydrase 3 and aspartate aminotransferase (P < 0·05). We suggest that the proteome changes in Longissimus muscle contributed to greater intramuscular lipid content in CLA-supplemented pigs.

Functional overload in ground squirrel plantaris muscle fails to induce myosin isoform shifts

We performed 2 wk of mechanical overload by synergist ablation on plantaris muscles from a small rodent hibernator, Spermophilus lateralis. While this muscle displays prominent myosin heavy-chain (MyHC) isoform shifts during hibernation, sensitivity to mechanical loading as a stimulus for muscle mass and isoform plasticity has not been demonstrated. Squirrel muscles, whether during hibernation or not, potentially are less sensitive to mechanical unloading, but we hypothesized that increased loading would produce the typical mammalian response of greater plantaris mass and MyHC shifts. Mechanical overload produced a 50% increase in muscle mass but, surprisingly, no changes in MyHC isoform protein or mRNA expression, despite previously observed fast-to-slow MyHC isoform switching during hibernation. Citrate synthase enzyme activity, as well as mRNA expression of creatine kinase and the muscle growth factor myostatin, were all unchanged. The mRNA expression of critical muscle atrophy genes decreased by 50% during hypertrophy, including ubiquitin ligases MuRF1 and MAFbx, and the related transcription factor FOXO-1a. Insulin-like growth factor (IGF-1) and hypoxia-inducible factor (HIF-1alpha) mRNA expression was elevated by 400% and 150%. Fast-to-slow MyHC isoform shifts appear unnecessary to support the increased recruitment of the plantaris muscle, shifts which are seen in other rodent models. Our results are consistent with muscular activity during interbout arousals as a potential mechanism to preserve muscle mass, but illustrate the primary importance of other seasonal factors besides patterns of muscle activation which must act in concert to alter MyHC isoforms and muscle fiber type during hibernation.

Skeletal muscle hexokinase: regulation in mammalian hibernation

Skeletal muscle hexokinase (HK) from Richardson's ground squirrels was analyzed to determine how the enzyme is regulated during hibernation, a state of cold torpor. The HK II isozyme dominated in muscle and ~15% of total HK was bound to the insoluble fraction. HK maximum activity was 33% lower in hibernator muscle and the enzyme showed a significantly higher K ( m ) ATP (by 80%) and a lower K ( i ) for glucose-6-P (by 40%) than euthermic HK (assayed at 22 degrees C). However, 5 degrees C assay significantly reduced K ( m ) glucose of hibernator HK. Stimulation of AMP-dependent protein kinase (AMPK) in hibernator extracts elevated the HK activity and reduced K ( m ) ATP, but did not affect euthermic HK. Stimulation of protein phosphatases significantly lowered the HK activity in both situations. AMPK-dependent phosphorylation was confirmed by immunopreciptiation of (32)P-labeled HK. DEAE-Sephadex ion exchange chromatography revealed two peaks of HK in hibernator muscle extracts (low and high phosphate forms), whereas only a single peak of phospho-HK was present in euthermic muscle. We conclude that differential control of muscle HK in euthermic versus hibernating states is derived from two main regulatory influences, reversible protein phosphorylation and temperature effects on kinetic properties.